Conventionally, a gallium nitride (GaN) compound semiconductor device (hereinafter called a GaN semiconductor element) is used as a semiconductor material in semiconductor elements for high-frequency devices. In a GaN semiconductor element, a buffer layer formed by using, for example, a Metal-Organic Chemical Vapor Deposition (MOCVD) method, and an electron traveling layer that is doped with impurities, are provided on the surface of a substrate. Recently, because it has been recognized that GaN semiconductor elements can be applied as well to semiconductor devices for power (power devices) in addition to high-frequency applications, studies are being carried out on GaN semiconductor elements that handle high withstand voltages and large currents.
A gallium nitride semiconductor element having a MOS structure is disclosed in Patent Document 1. A schematic structural view of the gallium nitride semiconductor element having a MOS structure that is disclosed in Patent Document 1 is shown in FIG. 21. As shown in FIG. 21, at a conventional gallium nitride semiconductor element 1000, a GaN layer 1016, that functions as an electron traveling layer, and an AlGaN layer 1020, that functions as an electron supplying layer, are layered on a substrate 1012 via a buffer layer 1014 for layering GaN crystals, and a heterojunction structure is formed. In the gallium nitride semiconductor of FIG. 21, two-dimensional electron gas (2DEG: hereinafter called 2DEG), that is formed directly beneath the interface of the GaN layer 1016 and the AlGaN layer 1020 (at the surface of the GaN layer 1016), is utilized as a carrier.
A recess portion 1021 is formed in a portion of the surface of the AlGaN layer 1020. A gate electrode 1028 is disposed via a gate insulating film 1022 in this recess portion 1021, and a MOS (n-type MOS) structure (a MOSFET portion) is structured.
When voltage is applied to the gate electrode 1028, electrons collect at the surface of the GaN layer 1016 that contacts the gate insulating film 1022, and a MOS channel is formed (enters into an on state), and the MOS channel is electrically connected to a 2DEG layer 1018 that is formed at the interface of the GaN layer 1016 and the AlGaN layer 1020, and a source electrode 1024 and a drain electrode 1026 enter into an electrically conductive state.
Further, in a case in which the MOS channel is in an off state, when voltage is applied between the source electrode 1024 and the drain electrode 1026, the 2DEG layer 1018 depletes from the gate end portion and a high withstand voltage can be maintained, and the gallium nitride semiconductor element 1000 functions as a high-power and high-withstand-voltage semiconductor element. Therefore, the development of nitride semiconductor elements as semiconductor elements for electric power, that are high-frequency and highly efficient, has advanced in recent years. Conventionally, devices that are called so-called HEMTs in which the gate portion is a Schottky junction have mainly been developed. Attention has been paid to such devices because the driving circuit is simpler at the insulating gate, and it is easy to use the device as a so-called normally-off device that is in an electrically off state when the gate voltage that is applied to the MOSFET portion is 0 V (when gate voltage is not applied).
Because the gallium nitride semiconductor element 1000 is used as a semiconductor element for electric power, there are the great advantages that the gallium nitride semiconductor element 1000 operates at high speed and the conduction resistance thereof is low. On the other hand, it has been learned that, when attempting to deplete the 2DEG layer 1018, there are cases in which the problem often arises that a large electrical field concentrates at a drain side end portion 1023 of the MOSFET portion and the gate insulating film 1022 is broken. It has been learned that the cause of this is that holes that are generated by the high electrical field concentrate at the gate insulating film 1022 and at the AlGaN layer 1020/GaN layer 1016 interface that is near to the gate insulating film 1022, and almost all of the voltage that is applied to the drain electrode 1026 is applied to the gate insulating film 1022.
Moreover, even if the gate insulating film 1022 is not broken, if a large voltage is continually applied to the drain electrode 1026 over a long period of time, there are cases in which there arises the problem of reliability such that a high electrical field is applied to the gate insulating film 1022 over a long period of time, and the characteristic thereof deteriorates over time.
In order to prevent this, it has been though to make the electron concentration of the 2DEG be a concentration of less than or equal to around 2×1012 cm−2. Due thereto, the 2DEG is easily depleted, and the effect of maintaining the withstand voltage is obtained. However, when the concentration of the 2DEG is lowered, the conduction resistance of the 2DEG layer 1018 portion becomes large, and therefore, the on resistance of the element on the whole rises, and there is the drawback that the intrinsic advantages of a nitride semiconductor are lost.
Further, an example of another means therefor is a means that is called a field plate at the drain side end portion of the gate electrode 1028, in which the gate electrode 1028 is extended on an insulating film that is thicker than the gate insulating film 1022, and the electrical field of the thin gate insulating film 1022 portion is moderated. However, it has become clear that, with this means, in a case in which the electron concentration of the 2DEG is greater than or equal to 3×1012 cm−2, it is difficult to protect the gate insulating film 1022.
Still further, an example of another means is a means in which, by making the GaN layer 1016 be p-type, the holes that collect at the periphery of the gate insulating film 1022 are discharged to the p-type region, and the 2DEG layer 1018 is made to be easily depleted. This means has the advantage that, as shown in Non-Patent Document 1 for example, it is easy to widen the depletion layer by controlling the accepter concentration, and a high withstand voltage can be achieved. However, it is generally difficult to form a p-type layer of gallium nitride, and further, it is extremely difficult to carry out concentration control at around 1×1017 cm−3. In particular, in cases in which the substrate 1012 is formed of silicon, it is difficult to obtain a p-type layer itself. Namely, an extremely limited range of concentration and selection of substrate crystal are required.
Further, in the structure of FIG. 21, because the source side and the drain side have a basically contrasting structure across the gate electrode 1028, there is no so-called freewheeling diode (hereinafter called FWD). Therefore, for example, if a nitride semiconductor element is used as an inverter or the like, a diode bearing the function of an FWD must be connected in parallel to the exterior of the nitride semiconductor element.
On the other hand, Patent Document 2 discloses a high-withstand-voltage power device that is a normally-on device and in which a so-called high-withstand-voltage JFET (Junction-Field-Effect-Transistor) and a low-withstand-voltage MOSFET are cascode-connected in series. The connected state of this JFET and MOSFET is shown in FIG. 22. This is a structure in which the gate terminal of the JFET short circuits with the source of the MOSFET that is connected in series, and, when viewed from the exterior, it is attempted to operate this power device as if it were an insulating gate device. Because a MOSFET can use a device that has low withstand voltage and on-resistance, the JFET is normally-on. However, an SiC MOSFET, that has high withstand voltage and low resistance, is connected to a low-withstand-voltage silicon MOSFET, and a device, that is normally-off and has low on-resistance and high withstand voltage, is realized.
On the other hand, for example, the nitride semiconductor element shown in Patent Document 3, such as shown in FIG. 23, is known. A semiconductor element 2000 shown in FIG. 23 is structured to include a reverse surface electrode 2035, a substrate 2012, a buffer layer 2014, an electron traveling layer 2016, a 2DEG layer 2018, an electron supplying layer 2020, an insulating film 2033, a source electrode 2024, a drain electrode 2026, a gate electrode 2028, and a Schottky electrode 2031. In the semiconductor element 2000, the source electrode 2024, the drain electrode 2026, the gate electrode 2028 and the Schottky electrode 2031 are formed directly on the electron supplying layer 2020. Due to the Schottky electrode 2031, that is provided between the drain electrode 2026 and the gate electrode 2028, short circuiting with the source electrode 2024, high-speed operation is realized. A recess is not formed in the gate electrode 2028 portion of this nitride semiconductor element, and the problem of a large electrical field concentrating at the drain side end portion 1023 in FIG. 21 and the gate insulating film 1022 being broken such as in Patent Document 1, does not arise. However, because the nitride semiconductor element shown in Patent Document 3 is a normally-on type, safety at the time of a failure cannot be ensured.
Patent Document 1: Pamphlet of International Publication No. 2003/071607
Patent Document 2: U.S. Pat. No. 6,900,537
Patent Document 3: Japanese Patent Application Laid-Open No. 2007-273795
Non-Patent Document 1: Proceedings of International Symposium on Power Semiconductor Device and IC's “Enhancement-mode GaN Hybrid MOS-HEMTs with Ron, sp of 20 mΩ-cm2” (2008) pp. 295-298